Standard library header <meta> (C++26)

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Diagnostics library

Memory management library

Metaprogramming library (C++11)

General utilities library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Strings library

Text processing library

Numerics library

Date and time library

Input/output library

Filesystem library (C++17)

Concurrency support library (C++11)

Execution control library (C++26)

Technical specifications

Symbols index

External libraries

 

Standard library headers

Language support <cfloat> <climits> <compare> (C++20) <contracts> (C++26) <coroutine> (C++20) <csetjmp> <csignal> <cstdarg> <cstddef> <cstdint> (C++11) <cstdlib> <exception> <initializer_list> (C++11)   <limits> <new> <source_location> (C++20) <stdfloat> (C++23) <typeinfo> <version> (C++20) Concepts <concepts> (C++20) Diagnostics <cassert> <cerrno> <debugging> (C++26) <stacktrace> (C++23) <stdexcept> <system_error> (C++11) Memory management <memory> <memory_resource> (C++17) <scoped_allocator> (C++11) Metaprogramming <meta> (C++26) <ratio> (C++11) <type_traits> (C++11)

General utilities <any> (C++17) <bitset> <bit> (C++20) <charconv> (C++17) <expected> (C++23) <format> (C++20) <functional> <optional> (C++17) <stdbit.h> (C++26) <tuple> (C++11) <typeindex> (C++11) <utility> <variant> (C++17) Containers <array> (C++11) <deque> <flat_map> (C++23) <flat_set> (C++23) <forward_list> (C++11) <hive> (C++26) <inplace_vector> (C++26)   <list> <map> <mdspan> (C++23) <queue> <set> <span> (C++20) <stack> <unordered_map> (C++11) <unordered_set> (C++11) <vector> Iterators <iterator> Ranges <generator> (C++23) <ranges> (C++20)

Algorithms <algorithm> <numeric> Strings <cctype> <cstring> <cuchar> (C++11) <cwchar> <cwctype> <string_view> (C++17) <string> Text processing <clocale> <codecvt> (C++11/17/26*) <locale> <regex> (C++11) <text_encoding> (C++26)   Numerics <cfenv> (C++11) <cmath> <complex> <linalg> (C++26) <numbers> (C++20) <random> (C++11) <simd> (C++26) <stdckdint.h> (C++26) <valarray> Time <chrono> (C++11) <ctime> C compatibility <ccomplex> (C++11/17/20*) <ciso646> (until C++20) <cstdalign> (C++11/17/20*) <cstdbool> (C++11/17/20*) <ctgmath> (C++11/17/20*)

Input/output <cinttypes> (C++11) <cstdio> <filesystem> (C++17) <fstream> <iomanip> <iosfwd> <iostream> <ios> <istream> <ostream> <print> (C++23) <spanstream> (C++23) <sstream> <streambuf> <strstream> (C++98/26*) <syncstream> (C++20) Concurrency support <atomic> (C++11) <barrier> (C++20) <condition_variable> (C++11) <future> (C++11) <hazard_pointer> (C++26) <latch> (C++20) <mutex> (C++11) <rcu> (C++26) <semaphore> (C++20) <shared_mutex> (C++14) <stdatomic.h> (C++23) <stop_token> (C++20) <thread> (C++11) Execution support <execution> (C++17)

 

Includes
<compare> (C++20)	Three-way comparison operator support
<initializer_list> (C++11)	std::initializer_list class template
Concepts
Defined in namespace std::meta
reflection_range (C++26)	specifies a range of reflection values (concept)
Types
Defined in namespace std::meta
info (C++26)	the type of reflection values (typedef)
meta::exception (C++26)	exception thrown by reflection functions (class)
operators (C++26)	identifies overloadable operators (enum)
access_context (C++26)	represents a context for access checking (class)
functions
Defined in namespace std
define_static_string (C++26)	promotes compile-time string to static storage, returning a pointer to the first character of the static string (function template)
define_static_array (C++26)	promotes compile-time array into static storage, returning a span of the static array (function template)
define_static_object (C++26)	promotes compile-time value into static storage, returning a pointer to the static object (function template)
Defined in namespace std::meta
reflect_constant_string (C++26)	promotes a compile-time string to static storage, returning a reflection representing the static string (function template)
reflect_constant_array (C++26)	promotes compile-time array into static storage, returning a reflection representing the static array (function template)

Synopsis

#include <compare>
#include <initializer_list>

namespace std {
  // checking string literals
  consteval bool is_string_literal(const char* p);
  consteval bool is_string_literal(const wchar_t* p);
  consteval bool is_string_literal(const char8_t* p);
  consteval bool is_string_literal(const char16_t* p);
  consteval bool is_string_literal(const char32_t* p);

  // promoting to static storage
  namespace meta {
    template<ranges::input_range R> consteval info reflect_constant_string(R&& r);
    template<ranges::input_range R> consteval info reflect_constant_array(R&& r);
  }
  template<ranges::input_range R>
  consteval const ranges::range_value_t<R>* define_static_string(R&& r);
  template<ranges::input_range R>
  consteval span<const ranges::range_value_t<R>, /* see description */>
  define_static_array(R&& r);
  template<class T> consteval const remove_cvref_t<T>* define_static_object(T&& r);
}

namespace std::meta {
  using info = decltype(^^::);

  // class exception
  class exception;

  // operator representations
  enum class operators
  {
    /* see description */;
  };
  using enum operators;
  consteval operators operator_of(info r);
  consteval string_view symbol_of(operators op);
  consteval u8string_view u8symbol_of(operators op);

  // reflection names and locations
  consteval bool has_identifier(info r);

  consteval string_view identifier_of(info r);
  consteval u8string_view u8identifier_of(info r);

  consteval string_view display_string_of(info r);
  consteval u8string_view u8display_string_of(info r);

  consteval source_location source_location_of(info r);

  // reflection queries
  consteval info type_of(info r);
  consteval info object_of(info r);
  consteval info constant_of(info r);

  consteval bool is_public(info r);
  consteval bool is_protected(info r);
  consteval bool is_private(info r);

  consteval bool is_virtual(info r);
  consteval bool is_pure_virtual(info r);
  consteval bool is_override(info r);
  consteval bool is_final(info r);

  consteval bool is_deleted(info r);
  consteval bool is_defaulted(info r);
  consteval bool is_user_provided(info r);
  consteval bool is_user_declared(info r);
  consteval bool is_explicit(info r);
  consteval bool is_noexcept(info r);

  consteval bool is_bit_field(info r);
  consteval bool is_enumerator(info r);
  consteval bool is_annotation(info r);

  consteval bool is_const(info r);
  consteval bool is_volatile(info r);
  consteval bool is_mutable_member(info r);
  consteval bool is_lvalue_reference_qualified(info r);
  consteval bool is_rvalue_reference_qualified(info r);

  consteval bool has_static_storage_duration(info r);
  consteval bool has_thread_storage_duration(info r);
  consteval bool has_automatic_storage_duration(info r);

  consteval bool has_internal_linkage(info r);
  consteval bool has_module_linkage(info r);
  consteval bool has_external_linkage(info r);
  consteval bool has_c_language_linkage(info r);
  consteval bool has_linkage(info r);

  consteval bool is_complete_type(info r);
  consteval bool is_enumerable_type(info r);

  consteval bool is_variable(info r);
  consteval bool is_type(info r);
  consteval bool is_namespace(info r);
  consteval bool is_type_alias(info r);
  consteval bool is_namespace_alias(info r);

  consteval bool is_function(info r);
  consteval bool is_conversion_function(info r);
  consteval bool is_operator_function(info r);
  consteval bool is_literal_operator(info r);
  consteval bool is_special_member_function(info r);
  consteval bool is_constructor(info r);
  consteval bool is_default_constructor(info r);
  consteval bool is_copy_constructor(info r);
  consteval bool is_move_constructor(info r);
  consteval bool is_assignment(info r);
  consteval bool is_copy_assignment(info r);
  consteval bool is_move_assignment(info r);
  consteval bool is_destructor(info r);

  consteval bool is_function_parameter(info r);
  consteval bool is_explicit_object_parameter(info r);
  consteval bool has_default_argument(info r);
  consteval bool is_vararg_function(info r);

  consteval bool is_template(info r);
  consteval bool is_function_template(info r);
  consteval bool is_variable_template(info r);
  consteval bool is_class_template(info r);
  consteval bool is_alias_template(info r);
  consteval bool is_conversion_function_template(info r);
  consteval bool is_operator_function_template(info r);
  consteval bool is_literal_operator_template(info r);
  consteval bool is_constructor_template(info r);
  consteval bool is_concept(info r);

  consteval bool is_value(info r);
  consteval bool is_object(info r);

  consteval bool is_structured_binding(info r);

  consteval bool is_class_member(info r);
  consteval bool is_namespace_member(info r);
  consteval bool is_nonstatic_data_member(info r);
  consteval bool is_static_member(info r);
  consteval bool is_base(info r);

  consteval bool has_default_member_initializer(info r);

  consteval bool has_parent(info r);
  consteval info parent_of(info r);

  consteval info dealias(info r);

  consteval bool has_template_arguments(info r);
  consteval info template_of(info r);
  consteval vector<info> template_arguments_of(info r);
  consteval vector<info> parameters_of(info r);
  consteval info variable_of(info r);
  consteval info return_type_of(info r);

  // access control context
  struct access_context;

  // member accessibility queries
  consteval bool is_accessible(info r, access_context ctx);
  consteval bool has_inaccessible_nonstatic_data_members(info r, access_context ctx);
  consteval bool has_inaccessible_bases(info r, access_context ctx);
  consteval bool has_inaccessible_subobjects(info r, access_context ctx);

  // scope identification
  consteval info current_function();
  consteval info current_class();
  consteval info current_namespace();

  // reflection member queries
  consteval vector<info> members_of(info r, access_context ctx);
  consteval vector<info> bases_of(info type, access_context ctx);
  consteval vector<info> static_data_members_of(info type, access_context ctx);
  consteval vector<info> nonstatic_data_members_of(info type, access_context ctx);
  consteval vector<info> subobjects_of(info type, access_context ctx);
  consteval vector<info> enumerators_of(info type_enum);

  // reflection layout queries
  struct member_offset;
  consteval member_offset offset_of(info r);
  consteval size_t size_of(info r);
  consteval size_t alignment_of(info r);
  consteval size_t bit_size_of(info r);

  // annotation reflection
  consteval vector<info> annotations_of(info item);
  consteval vector<info> annotations_of_with_type(info item, info type);

  // value extraction
  template<class T> consteval T extract(info r);

  // reflection substitution
  template<class R>
  concept reflection_range = /* see description */;

  template<reflection_range R = initializer_list<info>>
  consteval bool can_substitute(info templ, R&& arguments);
  template<reflection_range R = initializer_list<info>>
  consteval info substitute(info templ, R&& arguments);

  // expression result reflection
  template<class T> consteval info reflect_constant(T expr);
  template<class T> consteval info reflect_object(T& expr);
  template<class T> consteval info reflect_function(T& fn);

  // class definition generation
  struct data_member_options;
  consteval info data_member_spec(info type, data_member_options options);
  consteval bool is_data_member_spec(info r);
  template<reflection_range R = initializer_list<info>>
  consteval info define_aggregate(info type_class, R&& mdescrs);

  // primary type categories
  consteval bool is_void_type(info type);
  consteval bool is_null_pointer_type(info type);
  consteval bool is_integral_type(info type);
  consteval bool is_floating_point_type(info type);
  consteval bool is_array_type(info type);
  consteval bool is_pointer_type(info type);
  consteval bool is_lvalue_reference_type(info type);
  consteval bool is_rvalue_reference_type(info type);
  consteval bool is_member_object_pointer_type(info type);
  consteval bool is_member_function_pointer_type(info type);
  consteval bool is_enum_type(info type);
  consteval bool is_union_type(info type);
  consteval bool is_class_type(info type);
  consteval bool is_function_type(info type);
  consteval bool is_reflection_type(info type);

  // composite type categories
  consteval bool is_reference_type(info type);
  consteval bool is_arithmetic_type(info type);
  consteval bool is_fundamental_type(info type);
  consteval bool is_object_type(info type);
  consteval bool is_scalar_type(info type);
  consteval bool is_compound_type(info type);
  consteval bool is_member_pointer_type(info type);

  // type properties
  consteval bool is_const_type(info type);
  consteval bool is_volatile_type(info type);
  consteval bool is_trivially_copyable_type(info type);
  consteval bool is_standard_layout_type(info type);
  consteval bool is_empty_type(info type);
  consteval bool is_polymorphic_type(info type);
  consteval bool is_abstract_type(info type);
  consteval bool is_final_type(info type);
  consteval bool is_aggregate_type(info type);
  consteval bool is_structural_type(info type);
  consteval bool is_signed_type(info type);
  consteval bool is_unsigned_type(info type);
  consteval bool is_bounded_array_type(info type);
  consteval bool is_unbounded_array_type(info type);
  consteval bool is_scoped_enum_type(info type);

  template<reflection_range R = initializer_list<info>>
  consteval bool is_constructible_type(info type, R&& type_args);
  consteval bool is_default_constructible_type(info type);
  consteval bool is_copy_constructible_type(info type);
  consteval bool is_move_constructible_type(info type);

  consteval bool is_assignable_type(info type_dst, info type_src);
  consteval bool is_copy_assignable_type(info type);
  consteval bool is_move_assignable_type(info type);

  consteval bool is_swappable_with_type(info type1, info type2);
  consteval bool is_swappable_type(info type);

  consteval bool is_destructible_type(info type);

  template<reflection_range R = initializer_list<info>>
  consteval bool is_trivially_constructible_type(info type, R&& type_args);
  consteval bool is_trivially_default_constructible_type(info type);
  consteval bool is_trivially_copy_constructible_type(info type);
  consteval bool is_trivially_move_constructible_type(info type);

  consteval bool is_trivially_assignable_type(info type_dst, info type_src);
  consteval bool is_trivially_copy_assignable_type(info type);
  consteval bool is_trivially_move_assignable_type(info type);
  consteval bool is_trivially_destructible_type(info type);

  template<reflection_range R = initializer_list<info>>
  consteval bool is_nothrow_constructible_type(info type, R&& type_args);
  consteval bool is_nothrow_default_constructible_type(info type);
  consteval bool is_nothrow_copy_constructible_type(info type);
  consteval bool is_nothrow_move_constructible_type(info type);

  consteval bool is_nothrow_assignable_type(info type_dst, info type_src);
  consteval bool is_nothrow_copy_assignable_type(info type);
  consteval bool is_nothrow_move_assignable_type(info type);

  consteval bool is_nothrow_swappable_with_type(info type1, info type2);
  consteval bool is_nothrow_swappable_type(info type);

  consteval bool is_nothrow_destructible_type(info type);

  consteval bool is_implicit_lifetime_type(info type);

  consteval bool has_virtual_destructor(info type);

  consteval bool has_unique_object_representations(info type);

  consteval bool reference_constructs_from_temporary(info type_dst, info type_src);
  consteval bool reference_converts_from_temporary(info type_dst, info type_src);

  // type property queries
  consteval size_t rank(info type);
  consteval size_t extent(info type, unsigned i = 0);

  // type relations
  consteval bool is_same_type(info type1, info type2);
  consteval bool is_base_of_type(info type_base, info type_derived);
  consteval bool is_virtual_base_of_type(info type_base, info type_derived);
  consteval bool is_convertible_type(info type_src, info type_dst);
  consteval bool is_nothrow_convertible_type(info type_src, info type_dst);
  consteval bool is_layout_compatible_type(info type1, info type2);
  consteval bool is_pointer_interconvertible_base_of_type(info type_base,
                                                          info type_derived);

  template<reflection_range R = initializer_list<info>>
  consteval bool is_invocable_type(info type, R&& type_args);
  template<reflection_range R = initializer_list<info>>
  consteval bool is_invocable_r_type(info type_result, info type, R&& type_args);

  template<reflection_range R = initializer_list<info>>
  consteval bool is_nothrow_invocable_type(info type, R&& type_args);
  template<reflection_range R = initializer_list<info>>
  consteval bool is_nothrow_invocable_r_type(info type_result, info type, R&& type_args);

  // const-volatile modifications
  consteval info remove_const(info type);
  consteval info remove_volatile(info type);
  consteval info remove_cv(info type);
  consteval info add_const(info type);
  consteval info add_volatile(info type);
  consteval info add_cv(info type);

  // reference modifications
  consteval info remove_reference(info type);
  consteval info add_lvalue_reference(info type);
  consteval info add_rvalue_reference(info type);

  // sign modifications
  consteval info make_signed(info type);
  consteval info make_unsigned(info type);

  // array modifications
  consteval info remove_extent(info type);
  consteval info remove_all_extents(info type);

  // pointer modifications
  consteval info remove_pointer(info type);
  consteval info add_pointer(info type);

  // other transformations
  consteval info remove_cvref(info type);
  consteval info decay(info type);
  template<reflection_range R = initializer_list<info>>
  consteval info common_type(R&& type_args);
  template<reflection_range R = initializer_list<info>>
  consteval info common_reference(R&& type_args);
  consteval info underlying_type(info type);
  template<reflection_range R = initializer_list<info>>
  consteval info invoke_result(info type, R&& type_args);
  consteval info unwrap_reference(info type);
  consteval info unwrap_ref_decay(info type);

  consteval size_t tuple_size(info type);
  consteval info tuple_element(size_t index, info type);
  consteval bool is_applicable_type(info fn, info tuple);
  consteval bool is_nothrow_applicable_type(info fn, info tuple);
  consteval info apply_result(info fn, info tuple);

  consteval size_t variant_size(info type);
  consteval info variant_alternative(size_t index, info type);

  consteval strong_ordering type_order(info type_a, info type_b);
}

Class std::meta::exception

namespace std::meta {
  class exception : public std::exception
  {
  private:
    optional<string> /*what_*/; // exposition-only
    u8string /*u8what_*/;       // exposition-only
    info /*from_*/;             // exposition-only
    source_location /*where_*/; // exposition-only

  public:
    consteval exception(u8string_view what,
                        info from,
                        source_location where = source_location::current()) noexcept;

    consteval exception(string_view what,
                        info from,
                        source_location where = source_location::current()) noexcept;

    exception(const exception&) = default;
    exception(exception&&) = default;

    exception& operator=(const exception&) = default;
    exception& operator=(exception&&) = default;

    constexpr const char* what() const noexcept override;
    consteval u8string_view u8what() const noexcept;
    consteval info from() const noexcept;
    consteval source_location where() const noexcept;
  };
}

Class std::meta::access_context

namespace std::meta {
  struct access_context
  {
    access_context() = delete;

    consteval info scope() const;
    consteval info designating_class() const;

    static consteval access_context current() noexcept;
    static consteval access_context unprivileged() noexcept;
    static consteval access_context unchecked() noexcept;
    consteval access_context via(info cls) const;
  };
}

Class std::meta::member_offset

namespace std::meta {
  struct member_offset
  {
    ptrdiff_t bytes;
    ptrdiff_t bits;
    constexpr ptrdiff_t total_bits() const;
    auto operator<=>(const member_offset&) const = default;
  };
}

Class std::meta::data_member_options

namespace std::meta {
  struct data_member_options
  {
    struct /*name-type*/
    {                                         // exposition-only
      template<class T>
        requires constructible_from<u8string, T>
      consteval /*name-type*/(T&&);

      template<class T>
        requires constructible_from<string, T>
      consteval /*name-type*/(T&&);

    private:
      variant<u8string, string> /*contents*/; // exposition-only
    };

    optional</*name-type*/> name;
    optional<int> alignment;
    optional<int> bit_width;
    bool no_unique_address = false;
    vector<info> annotations;
  };
}